The present invention is related to a process for making strain-hardened polymer products using polymer-nanoparticle compositions.
Strain hardening is the most important characteristic in the production and usability of polymeric films in such processes as Tenter frame biaxial film stretching, blow molding, film blowing, thermoforming, and the like. This property affects structural characteristics such as crystallization as well as thickness uniformity of polymers. It is at this point that many polymers are known to crystallize due to orientation from deformation, with a consequential affect on optical, physical and mechanical properties.
The reasoning for controlling strain hardening is due to its effect on structural development in materials. The net effect of this behavior translates into “self-leveling” that allows thinned portions of a material to sustain higher forces while transferring the deformation to other undeformed regions, resulting in achieving greater uniformity in the thickness of the products. For this reason, it is of critical importance to find an efficient means of selectively controlling the strain hardening behavior of polymers. As an example, in film production for such industries as information recording (audio and video cassettes, etc.), film uniformity and lack of surface roughness are of paramount importance for acceptable products.
There have been many attempts to control the strain hardening behavior of polymeric films through the addition of additives, by blending, or by co-polymerization. But, adding fillers does not necessarily improve mechanical and physical properties of polymers, and studies indicate that they may not have desired effect on the deformation behavior. For example, the work of Taniguchi et al (Atsushi Taniguchi and Miko Cakmak, “The effect of titanium dioxide particles on the deformation behavior and orientation development in PET films”, Annual Technical Conference—Society of Plastics Engineers (2000), 58th (Vol. 2), 1786-1790) showed that adding submicron sized particles retards the strain hardening process to higher strain levels. Tanaguchi et al. reported that the effect of submicron size TiO2 particles at varying concentrations on the stress-strain behavior of uniaxially deformed PET films from the amorphous state. The TiO2 particles act as nucleation agents and enhance the thermally induced crystallization of PET. When stretched from the amorphous state, TiO2 particles at concentrations as low as 0.35 percent reduce the overall stress and delay strain hardening, thereby hindering orientation induced crystallization. Consequently, films stretched under the same conditions, but containing higher levels of TiO2 have both lower crystallinity and orientation. They attribute this behavior to the reductions in the number of chain entanglements due to the presence of small amounts of TiO2 particles.
Iwakura, et al. (Iwakura, K.; Wang, Y. D.; and Cakmak, M., “Effect of biaxial stretching on thickness uniformity and surface roughness of PET and PPS films,” Int. Polym. Process. (1992), 7(4), 327-333) in their publication performed biaxial film-stretching studies with poly(ethylene terephthalate) (PET) and poly(p-phenylene sulfide) (PPS) to observe the effects of stretching conditions on film properties; particularly surface smoothness and thickness uniformity. They found that by decreasing the stretching temperature and increasing the stretch ratio, they could improve these properties in PET, but not in PPS. They also observed that the most important factor in controlling these properties was the strain hardening mechanism. If this could be controlled, so too would the thickness uniformity and surface smoothness. Under the conditions used in these experiments, strain hardening occurred for PET, but not for PPS, and once strain hardening was attained, properties improved drastically, especially the thickness uniformity. They attributed the problems in PPS to branching of the polymer chains.
It would be a very easy to process films to have desirable properties if all that one had to do was find the strain hardening point, and just stretch beyond this. However, this is not always possible. Sometimes the strain hardening point occurs at too high of a strain to be feasible, or in some other cases, strain hardening is just not possible under normal processing conditions. In addition, there are other side issues in processing and properties. Characteristics, such as miscibility and domain size, have great effect on polymer properties. In the instance where strain hardening can be improved, other properties may suffer. A modification that works very well in one case can yield a very poor result in another, or too much modification may actually be detrimental to some properties. In other cases, a modification may work very well, but the strain hardening behavior still might not be very well controlled, as the modification might work only to a certain extent.